Safety heat control system



March 6, 1951 R. PRATT 2,544,511

SAFETY HEAT CONTROL SYSTEM Filed Sept. 30, 1944 2 SheetsSheet l Help 5 w/ rc/r /8 Firra QNE/S.

March 6, 1951 R. PRATT 2,544,511

SAFETY HEAT CONTROL SYSTEM Filed Sept. 50, 1944 2 Sheets-Sheet 2 7-le/p sun-cu M/mvram RYDER PRA 1-7;

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HTTOF Patented Mar. 6, 1951 UNITED STATES PATENT OFFICE Application September 30, 1944, Serial No. 556,516

15 Claims. (01. 158-28) The present invention relates to a heat control device. It is especially adapted to the control of burners requiring ignition, but it will be evident that in some aspects it will be applicable to other heat control or refrigeration control.

An object of the invention is to provide a control that is simple and inexpensive, yet safe. More particularly, it is an object to provide a control with a limited starting period followed by a running period, only if combustion occurs, and with safety features including a scavenging period, the whole being attained through the use of delayed action relays.

A further object is to provide a control of the foregoing kind in which momentary power failure with no igniter in operation cannot result in an immediate restart of the burner, but restart is required to await completion of a scavenging period.

A specific object is to provide a control having a timed starting circuit, a running circuit, and means in the running circuit to open the starting circuit and hold it open for a period after the running circuit opens.

A further object is to provide in a control of the foregoing kind an ignition means that is cut out upon combustion, or, particularly, with ignition means that is cut out only after a safety means has been made operative to prevent opcontrol;

Fig. 2 is a schematic view of a second type of such control; and

Fig. 3 is a schematic view of a third and most desirable type of such control.

Referring to Fig. 1,. electric power lines are shown at and II. These are bridged by the primary of a transformer T. The control circuit is operated principally off the secondary or low voltage side of the transformer. It includes a room or similar space thermostat I2 having, if desired, an anticipating heater I3. The anticipating heater is connected by lines l4 and through the bimetal switch 16 of the heat relay; thence is connected by a line IT to the bimetal tit switch I8 of a second heat relay. From the switch [8, a line I9 leads through the heating coil 20 that opens the second heat relay switch 18 after the heater 2!] has been energized a predetermined period of time. The heater 20 is connected by lines 23 and 24 through a main relay coil 25. From this coil, 2. line 26 leads back into the secondary of the transformer T.

A running circuit around the second heat relay switch [3 is provided by a line 30 connected to the line I 4 and leading into a heater 3| for the first heat relay switch [6. From the heater 3|, a line 32 leads through a bucking relay coil 33 associated with the main relay coil 25. From the bucking relay coil 33, a line 34 leads through one set of contacts 31 of a combustion safety switch 35. From the combustion safety switch, a line 36 is connected back to the line 24. The combustion switch is one that closes the contacts 31 only when combustion exists, and alternately closes the contacts 38 only when the burner is cold.

The main magnetic relay operates a switch 40, one side of which is connected to the main power line 10. On the other side of the switch a line 4| leads through the burner device 42, which is, in turn, connected by a line43 to the other power line H. The burner device is, typically, a power driven mechanism supplying fuel, or air, or both, to a furnace.

A line 45 is connected to the line 4| and leads to cold contacts 38 on the combustion safety switch 35. These contacts are in series, by means of a line 16, with the igniter 41 that is, in turn, connected to the power line H.

The room thermostat I2 is disposed in the room or other space the temperature of which is to be controlled. The burner motor or burner device 42 represents an electrical burner operating means supplying fuel, or air, or both, that requires a short period during which combustion ordinarily occurs. The heat-responsive elements of the combustion safety switch 35 are located at some point so as to be quickly responsive to the presence or absence of combustion in the furnace.

Operation of Fig. 1

When the room thermostat l2 closes its switch in response to a demand for heat, the circuit is closed from the secondary of the transformer through the thermostat 12, the anticipating heater l3 (if used) the line H, the line IS, the switch I6, the line H, the switch l8, the line 13, the relay heater 20, the line 21, the line 23,

3 the line 24, the main relay coil 25, and the line 25 back to the secondary.

When the coil 25 is thus put in circuit and energized, it pulls up the switch 40, closing the lines it] and M so that the burner device is put in operation across the lines It and II. At the same time, and as long as the combustion safety switch 35 is cold, the igniter 41 is put in circuit, by the line 4 I, through the line 45, the combustion safety switch contacts 38, and the line 46. With both the burner motor and the igniter in operation, the former supplying the proper fuel mixture, ignition will normally take place.

During this phase of the cycle, the relay heater 2G is generating heat for the bimetal switch blade I8. After a predetermined lapse of time, sufficient heat will be applied to the bimetal l8 to cause it to open. This period of time is fixed at greater than the normal time for combustion to occur in the furnace.

When combustion occurs, the combustion safety switch 35 responds, shifting its switch to open the ignition circuit contacts 38 and to close the running circuit contacts 31. This latter effects a circuit around the heater 20 of the second heat relay as follows: The line I4, the line 30, the heater 3! of the first heat relay, the line 32-, the bucking coil 33, the line 34, the combustion safety switch 35, the line 36 to the line 24, whence the circuit completes itself through the main relay coil 25.

The foregoing circuit not only shunts the second heat relay heater 20, but also energizes the first heat relay heater 3| and the bucking coil 33. Preferably the design is such that when the running circuit is closed, the current through the second heater 2G is reduced so that the time to open the heat relay I8 is increased.

The application of current to the. heater 3| causes it to generate heat and apply it to the bimetal switch H5. After a short period, the switch it is opened, and it is held open during the remainder of the running period so long as the combustion safety switch is in its hot position.

The bucking coil 33 is of insuflicient strength to cause the relay 25 to open the switch 40, but it is of sufficient strength to prevent the main coil 25 from pulling the relay closed when both coils are in series, that is with the starting circuit open and the running circuit closed.

In the normal cycle, the running period continues until the room thermostat l2 opens, deenergizing both coils 25 and 33, and opening the switch 40. It is impossible, however, to complete the starting circuit immediately thereafter because the bimetal [6 must cool before the relay 25 can be reenergized and close the switch 4!]. Even if there is a reclosing of the room thermostat after combustion ceases, but before the combustion safety switch cools, the burner motor will not start because the switch 46 remains open. Although the running circuit might thus reclose, both the bucking coil and the main coil are energized in series, with the result that insufiicient magnetic pull is generated to draw in the switch 48. The result of such false start is to apply further heat to the bimetal 16 from the coil 3 l, until the combustion safety switch 35 cools and opens due to the absence of combustion. As it does cool and shifts to its lower position in the drawing, the running circuit through the contacts 3'! is broken, and, as soon as the switch l6 closes, the starting circuit will again be ready for restart under control of the room thermostat.

In the event that power fails momentarily during a starting eriod before the stack switch 35 has heated, the main relay will drop out but may reclose again upon restoration of power, to continue the starting cycle.

If power fails momentarily during a running period, the relay will drop out, stopping the burner motor, which is at this time operating without. the operation of the igniter. This condition is apt to cause flame failure so that a restart of the burner motor without the igniter would be dangerous. However, such restart is prevented upon. restoration of power because the main coil circuit is open at the switch H5, in which condition, with bothcoils 25 and 33 in series, the relay has insuflicient power to pull the switch 40 closed. Therefore, upon such momentary power failure during the running period, the burner motor will remain out of operation until the combustion safety switch cools to break the running circuit, and toreestablish the starting circuit.

If, through any circumstance, the starting circuit through the switches 16 and i8 remains energized, as where combustion does not occur, the heater 2!] will continue to generate heat, and finally will open the switch I8, which switch is one requiring manual reset.

If, for any reason, there is a failure of the running circuit, as when combustion fails after an operation has been effected, the result will simply be a temporary shut-down, because the switch contacts 31 will open, and the starting circuit will be opened at the switch I6. This switch 15 will remain open for a scavenging period until the heater 3| cools, and a new start can commence. If flame failure occurs before the switch I5 has opened, the starting circuit will be re opened shortly by the reestablished heating of the heater 26, which will open the switch I8.

The modification of Fig. 2

In Fig. 2, a similar result is obtained by the use of a resistor in place of the bucking coil. It will be seen that the resistor 33a is substituted for the bucking coil 33, and that otherwise the circuits are the same. When the combustion safety switch 35 heats, it establishes a running, circuit around the starting circuit through the switches l6 and [8, the heater 20 andthe resistor 22. The resistor 33a in the running circuit cuts down the current through the main coil 25 to such a point that the coil 25 may hold the switch :5 closed but cannot reclose the relay in the event it drops out when the starting circuit is open.

The modification of Fig. 3

The preferred form of the invention is shown in Fig. 3 and includes additional safety features. In it, the two power lines are shown at 53 and 5|. The transformer primary is connected across them. In the secondary circuit, there is a room thermostat 52 which can have an anticipating heater 53, from which there leads a line 54. This is connected with a line 55 leading to a first bimetal switch 56. The line 51 leads from the switch 56 to a heater 60. From this heater, 2. line 6! leads through a bimetal switch 62. A line 63 leads from the switch 62 into the main coil 64 of the main relay, the other end of which is connected by a line 65 back to the secondary.

In this case, a running circuit comprises a line 28 connected to the line. 54. The line 18 is connected with a bucking coil H on the relay. A line i2 leads from the bucking coil through a heater 13-, from which a line H leads into the combustion safety switch 15 connected by a line 15 to the switch 62.

The main relay in this case controls two switches Bi) and 8|, both of which are connected to the main power line 50. The switch 80 is also connected by a line 82 through a bimetal switch 83 associated with theheater 13. A line 84 connects the switch 83 to the igniter device 85, which is, in turn, connected by a line 86 with the other power line 5!.

The switch SI connects the line 56 through a line 90 with the burner device 9 l which, in turn, is connected to the line 5|.

. In this control, the heater l3 acts upon both of the bimetal switches 56 and 83. When this heater is hot, it opens both switches simultaneously. The heater so is the warp switch timer, and opens the bimetal switch 62 after a predetermined interval of time.

Operation of Fig. 3

When the room thermostat 52 closes upon a demand for heat, a circuit is completed from the secondary through the thermostat 52, the anticipating heater 53 (if used), the line 5%, the line 55, the now closed switch 56, the line 5i, the heater the line Bl, the now closed heater switch 62, the line 63, the main relay coil 64, and the line 65 back to the secondary.

When the main coil 64 is energized, it closes the switches 80 and BI. Closure of the switch 88 puts the igniter device 85 in circuit through the presently closed bimetal switch 35. Closure of the switch 8| puts the burner device directly across the lines. Thereupon, fuel is supplied and ignited.

Combustion causes the combustion safety switch 15 to close after a brief interval. This closes the running circuit around the timing switch of the heater 68 through the line Hi, the bucking coil 1 I, the line '12, the heater K3, the line '54, the heated and closed combustion safety switch '55, the line 75, and thence through the switch 62 and the main coil 63 i,

The resistances in the starting circuit and running circuit are substantially equal so that when both circuits are closed the current in the starting circuit is somewhat reduced, thereby increasing the time required for the heater to to open the switch 52. In normal operation, when both starting and running circuits are closed, the heater F3 in the running circuit will open the switches 53 and 83 before the heater 60 opens the switch 62.

The opening of the switches 56 and 83 opens the starting and ignition circuits, leaving the main coil and bucking coil in series. As in connection with the control shown in Fig. 1, energization of the bucking coil produces insufficient force in opposition to the main coil 64 to throw out the relay, but it will prevent its being drawn in from an open position.

Whenthe switch 55 opens, the starting circuit is opened and the heater 6!! completely deenergized. Furthermore, as before, the heater l3 remains hot throughout the running period and between closing of the running'circuit and open-'' ing of the starting circuit at the switch 56, will be harmless, as restoration of power will occur with the igniter in circuit. A lock-out may occur if the switch 62 opens before a reignition can take place, but this is a safe condition, and only a manual reset is required to restart the mechanism. Manual reset cannot be effective until the heater to cools, so that scavenging is provided.

Momentary power failure, or other failure resulting in flame failure during the running period, is the same as opening the running circuit at the room thermostat. The bucking coil is in series, so that the switch as is held open until the switch 56 cools and recloses.

In all of these controls, inadvertent opening or vibrating of the room thermostat acts like a power failure. A snap-action thermostat is preferred.

The control in Fig. 3 will lock out if the timing switch heater 6G is held in circuit an excessive period of time through failure of combustion or if the switch 5% fails to open. The circuit through the bucking coil is also broken and the entire mechanism is maintained in a state of complete deenergization until manually reset.

acts to prevent an immediate recycle should the What is claimed is:

1. In a control for use with a burner operating device and a power source, the combination of a relay comprising a coil and a relay switch, the relay switch being normally in a first position, but displaceable into a second position by energization of the coil, the relay requiring more coil energy to displace the switch than to hold the switch in displaced position, said coil having two terminal connections, and developing magnetic power greater than the displacement-resisting load of the switch, whereby the coil is capable of displacing the relay switch to second position when a predetermined potential is applied to said terminals, power reducing means having terminal connections at its ends, said power reducing means being adapted to reduce the magnetic power of the coil to below that required to overcome the displacement-resisting load of the latter being in series with the other components of the holding circuit branch whereby the coil may be energized to close the relay switch when the closing circuit branch is energized, the coil may be held energized when the holding circuit branch is energized, and the closing circuit branch may be opened by energization of the holding circuit branch to leave the coil energizedat reduced magnetic power.

2. In a control for use with a burner operating device and a power source, the combination of a relay comprising a coil and a relay switch, the relay switch being normally in a first position, but displaceable into a second position by energization of the coil, the relay requiring more coil energy to displace the switch than to hold the switch in displaced position, said coil having two terminal connections, and developing magnetic power greater than the displacement-resist- 7, ing. load of the switch, whereby the; coil is capable of displacing the relay switch to second position when. a predetermined potential is applied to: said terminals, power reducing means having terminal connections at its ends, said power reducing means being adapted to reduce the magnetic power of the coil to. below that required to over come the displacement-resisting load of the switch but above the lesser load tending to re-- turn the. switch to its first position, a control switch device having a control switch and electrically energizable means to open the switch when the said means is energized and to permit the switch. to close when said means is denergized, and an additional switch; a closing circuit branch including the control switch and the coil in series, a holding circuit branch including the control switch energizable means, the power reducing means, the additional switch and the coil; the latter being in series with the other components of the holding circuit branch whereby the coil may be energized to close the relay switch when the closing circuit branch is energized, the coil may be held energized when the holding circuit branch is energized, and the closing circuit branch may be opened by energization of the holding circuit branch to leave the coil energized at reduced magnetic power, the additional switch comprising a combustion-responsive switch that is automatically closed when subjected to combustion temperatures and opened when not sub-- jected to combustion temperatures.

3. In a control for use with a burner operating device and a power source, the combination of a relay comprising a coil and a relay switch, the relay switch being normally in a first position, but displaceable into a second position by energization of the coil, the relay requiring more coil energy to displace the switch than to hold the switch in displaced position, said coil having two terminal connections, and developing magnetic power greater than the displacement-resisting load of the switch, whereby the coil is capable of displacing the relay switch to second position when a predetermined potential is applied to said terminals, power reducing means having terminal connections at its ends, said power reducing means being adapted to reduce the magnetic power of the coil to below that required to overcome the displacement-resisting load of the switch but above the lesser l'o'ad tending to return the switch to itsfirst position, a control switch device having a control switch and electrically energizable means to open the switch when the said means is energized and to permit the switch to close when said means is deenergized, and an additional switch; a closing circuit branch including the control switch and the coil in series, a holding circuit branch including the control switch energizable means, the power reducing means, the additional switch and the coil; the latter being in series with the other components of the holding circuit branch whereby the coil may be energized to close the relay switch when the closing circuit branch is energized, the cell may be held energized when the holding circuit branch is energized, and the closing circuit branch may be opened by energization of the holding circuit branch to leave the coil energized at reduced magnetic power, the control switch device comprising a time-delay relay in which the control switch does not reclose until after lapse of a time interval, upon deenergization of its energizable means.

4. In a control for use with a burner operating device and a power source, the combination of a relay comprising a coil and a relay switch, the relay switch being normally in a first position, but displaceable into a second position by energization of the coil, the relay requiring more coil energy to displace the switch than to hold the switch in displaced position, said coil having two terminal connections, and developing magnetic power greater than the displacement-resisting load of the switch, whereby the coil is capable of displacing the relay switch to second position when a predetermined potential is applied to said terminals, power reducing means having terminal connections at its ends, said power reducing means being adapted to reduce the magnetic power of the coil to below that required to overcome the displacement-resisting load of the switch but above the lesser load tending to return the switch to its first position, a control switch device having a control switch and electrically energizable means to open the switch when the said means is energized and to permit the switch to close when said means is deenergized, and an additional switch; a closing circuit branch including the control switch and the coil in series; a holding circuit branch including the control switch energizable means, the power reducing means, the additional switch and the coil, the latter being in series with the other components of the holding circuit branch; whereby the coil may be energized to close the relay switch when the closing circuit branch is energized, the coil may be held energized when the holding circuit branch is energized, and the closing circuit branch may be opened by energization of the holding circuit branch to leave the coil energized at reduced magnetic power, the additional switch comprising a combustion-responsive switch that is automatically closed when subjected to com bustion temperatures and opened when not subjected to combustion temperatures, and a space thermostat switch connected in series with both the closing circuit branch and the holding circuit branch.

5. In a control for use with a burner operating device and a burner, a power source, a space thermostat, and a combustion responsive switch having hot contacts closedin response to combustion, the combination of a relay coil and a relay switch operated by the coil for actuating the burner operating device, a time delay switch and a time delay operating means adapted to open the switch after an interval of energization thereof, said time delay switch and its operating means and the relay coil being connected together in a starting circuit branch; electrically energizable power reducing means to reduce the magnetic power of the relay coil to a value below that required to operate the switch but above that required to hold it operated, said power reducing means being connected with the relay coil to form a running circuit branch, said running circuit branch thus being partially interconnected with the starting circuit branch but excluding said time delay operating means; said starting circuit branch and running circuit branch being adapted for connection through the space thermostat, and the running circuit branch being adapted for connection through the hot contacts of the combustion switch to be energized when combustion exists, mechanism for rendering the starting circuit branch ineffective, including means to open the starting circuit branch and operating means for the opening means said opening means being connected in the starting 9 circuit branch and the operating means being connected in the running circuit branch, said running circuit branch, when energized alone, rendering the relay coil incapable of operating the relay switch but capable of holding it operated.

6. In a control for use with a burner operating device and a burner, a power source, a space thermostat, and a combustion responsive switch having hot contacts closed in response to combustion, the combination of a relay coil and a relay switch operated by the coil for actuatin the burner operating device, a-time delay switch and a time delay operating means adapted to open the switch after an interval of energization thereof, saidtime delay switch and its operating means and the relay coil being connected together in a starting circuit branch; a power reducing means to reduce the power of the relay coil to a value below that required to operate the switch but above that required to hold it operated, said power reducing means being connected with the relay coil to form a running circuit branch, said running circuit branch thus beingpartially interconnected with the starting circuit branch but excluding said time delay operating means; said starting circuit branch and running circuit branch being adapted for connection through the space thermostat, and the runing circuit branch being adapted for connection through the hot contacts of the combustion switch to be energized when combustion exists, mechanism for rendering the starting circuit branch ineffective, including means to open the starting circuit branch and operating means for the opening means, said opening means being connected in the starting circuit branch and the operating means being connected in the running circuit branch, said mechanism for rendering the starting circuit inelfective being constructed to delay reclosing of the opening means for a period after deenergization of the running circuit branch.

7 In a control for use with a burner operating device and an igniter, the combination of. a relay coil and a switch operated thereby to efiect operation of the burner operating device, a starting circuit branch for the relay, and a running circuit branch for the relay, said starting circuit branch having a control switch therein displaceable for rendering the starting circuit branch inelfective,

an igniter switch for controlling the igniter, and displaceable for rendering the igniter ineffective, and a switch operating means in the running circuit branch adapted to displace both the control switch and the igniter switch when the run ning circuit branch is ener ized.

8. In a control for use with a burner operating device and an igniter, the combination of a relay coil and a switch operated thereby to effect operation of the burner operating device, a starting circuit branch for the relay, and a running circuit branch for the relay, said starting circuit branch having a control switch therein displaceable for rendering the starting circuitbranch ineifective, an igniter switch for controlling the igniter, and displaceable for rendering the igniter inefiective, switch operating means in the running circuit branch adapted to displace both the control switch and the igniter switch when the running circuit branch is energized, and time delay means in the starting circuit branch for rendering the same ineffective to operate the relay after a predetermined period of energization thereof.

9. In a control for use with a burner operating device and an igniter, the combination'of a relay coil and a switch operated thereby to effect operation of the burner operating device, a starting circuit branch for the relay, and a running circuit branch for the relay, said starting circuit branch having a control switch therein displace.- able for rendering the starting circuit branch ineffective, an igniter switch for controlling the igniter, and displaceable for rendering the igniter inefiective, switch operating means in the running circuit branch adapted to displace both the control switch and the igniter switch when the running circuit branch is energized, and a combustion responsive switch in the running circuit branch, and adapted to prevent energization thereof in absence of combustion.

10. In a control for use with a burner operating device and an igniter, a relay coil and arelay switch operated thereby, the switch being adapted to be connected with the burner operating device, a time delay switch and operating means to open the switch after a predetermined time, an igniter switch, adapted when opened to render the igniter ineffective, a control switch, energizable means to open both the control switch and the igniter switch, a power reducing coil, terminal connections for connection with the combustion responsive switch, the control switch, the time delay switch, the time delay operating means and the first coil being connected in series to form a starting circuit means through which the relay coil may be energized and the relay switch operated, the energizable means, the power reducing coil, the relay coil, and the combustion responsive switch terminal connections being connected in series to form a running circuit means through which the relay coil may be held energized, the igniter switch opened and the control switch opened to render the starting circuit means ineffective, the power reducing coil being capable of reducing the power of the relay coil to between that required to operate the switch and that ree quired to hold it operated when the running circuit means is energized.

11. In a control for use with a burner operating device and an igniter, a relay coil and a relay switch operated thereby, the switch being adapted to be connected with the burner operating device, a time delay switch and operating means to open the switch after a predetermined time, an igniter switch, adapted when opened to render the igniter ineffective, a control switch, energizable means to open both the control switch and the igniter switch, a power reducing coil, terminal connections for connection with the combustion responsive switch, the control switch, the time delay switch, the time delay switch operating means and the coil being connected in series to form a starting circuit means through which the relay coil may be energized and the relay switch operated, the energizable means, the power reducing coil, the relay coil, and the combustion responsive switch terminal connections being connected in series to form a running circuit means through which the relay coil may be held energized, the igniter switch opened and the control switch opened to render the starting circuit means ineffective, the power reducing coil being capable of reducing the power of the relay coil to between that required to operate the switch and that required to hold it operated when the running circult means is energized, said energizable means comprising a heater, and the control switch including a heat responsive means, whereby when the running circuit means is deenergized, the

'11 control switch may 'not reclose until .it has undergone an interval of cooling.

1-2. .In a control mechanism, a .relay having a coil, and an actuating device operated to oper- .ative position upon energization of the coil, a timing device including a switch and time-delay operating means energizable to open the switch, a starting circuit branch for the relay coil, the branch interconnecting the timing device switch .and the time-delay operating means with the relay coil, whereby upon energization of the branch the relay coil is energized, but the coil is subject to deenergization upon lapse of a predetermined interval by the timing device, energyreducing means to render the coil incapable of .operating its actuated device to operative position, but not to render the :coil incapable of holding the device in said position, means to open the starting circuit branch including a switch in said branch, and energizable means to operate the switch, and a running circuit branch for the relay coil interconnecting the energy-reducing means and the energizable means of the switch to open the starting circuit branch with the relay coil.

13. The combination of claim 12, wherein the means to open the starting circuit branch is a time-delay device that maintains its switch in the starting branch open for a period after the energizable means to open the switch is deener glzed.

14. In a control mechanism, a relay having a coil adapted to pull in an actuating device, a time delay device including a switch and an electrical operating means therefor, the operating means being energizable to open the switch, and deenergiza'ble to permit closure thereof, a first circuit branch interconnecting the coil and the switch so that the coil may be energized thereby only when the switch is in closed position, means to reduce the magnetic pull of the coil to a point between that to pull in the actuating device and that to hold it in, a second circuit branch interconnecting the coil, the magnetic pull reducing means and the means to operate the switch, said 12 last-named means and its switch comprising a time-delay device of the type in which the switch is held against movement for a period after deenergization of its operating means, and then is moved.

15. In a control mechanism, a relay having a coil adapted to pull in an actuating device, a switch and an electrical operating means therefor, a first circuit branch interconnecting the coil and the switch so that the coil may be energized thereby only when the switch is in closed position, means to reduce the magnetic pull of the coil to a point between that to pull in the actuating device and that to hold .it in, a second circuit branch interconnecting the coil, the magnetic pull reducing means and the means to operate the switch, said switch operating means comprising a heater and its switch including a heat-responsive device, and both together comprising a time-delay device that holds the switch open for a period after deenergization of the heater.

RYDER PRATT.

REFERENCES CITED The following references are of record in the file of this patent:

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